Wikipedia says Phosh is based on the GTK toolkit and is in fact a fork of GNOME shell. Even their own webpage says so at https://phosh.mobi/about/
You are misreading the Phosh page. It says it uses a custom wlroots compositor on that page. You can see the source yourself: https://gitlab.gnome.org/World/Phosh/phoc
Yes it uses the GTK toolkit but that has nothing to do withe GNOME-Shell or its performance problems.